It is generally known to use springs as biasing elements. In many devices, springs undergo multiple load and recovery cycles over a useful life. During an individual cycle the spring is generally compressed by application of a displacement force. This displacement force may be a compressive force or a tensioning force. As the displacement force is applied, the spring acts to counter the displacement force by biasing back towards the original position.
In general, during each individual cycle a spring substantially acts according to Hook's law of elasticity which requires that the distance of the extension or compression is proportional to the restoring force exerted by the spring. This is represented by the following formula:F=−KX where “X” is the distance that the spring has been stretched or compressed away from its equilibrium position, “F” is the restoring force exerted by the spring and “K” is the force constant (also known as the spring constant) of the spring which normally has units of force per unit length. The negative sign reflects that the restoring force is in opposition to the direction of displacement. The potential energy stored by the extended or compressed spring is given by the formula:U=½KX2 which is the integral of force over distance.
Over an extended life of multiple load and recovery cycles the force constant may tend to gradually degrade over time due to deformation and/or crystalline structural changes within the material forming the spring. As will be appreciated from the above formulas, such degradation in the force constant likewise causes a reduction in the restorative force and potential energy provided by the spring. When these levels become too low, the spring is no longer functional for its purpose of counterbalancing a displacement force and the spring must be replaced. In the past, springs with degraded force constants were often discarded and replaced with new springs. However, due to the cost and effort associated with manufacturing an original precision spring, this practice may be undesirable.
U.S. Pat. No. 6,101,718 to Zysman issued Aug. 15, 2007, discloses a method for producing a mattress with improved load bearing capacity. The method disclosed in U.S. Pat. No. 6,101,718 uses a stripped inner spring assembly which is repaired and heat treated to provide stress relief to components with little or no prior heat treating history. The inner spring assembly is then recovered with padding and ticking. The resultant mattress has an overall improved load bearing capacity relative to the original mattress when it was new.
In the mattress construction disclosed in U.S. Pat. No. 6,101,718 the spring elements are subjected to a stress relief treatment for a relatively brief time of about ten to twenty minutes at a relatively low temperature of about 550 degrees Fahrenheit. Such a treatment is intended to relieve stresses in components which had little if any stress relief at the time of original assembly. U.S. Pat. No. 6,101,718 notes that if temperatures are too high the wire forming the spring elements may undergo degradation.
While a system such as disclosed in U.S. Pat. No. 6,101,718 may provide certain stress relief benefits to elements which have not previously been subjected to heat treatment, the advocated low temperatures and short durations of the treatment may not be adequate to provide significant structural long term recovery to elements subject to degradation. Accordingly, an improved system for the salvage of springs suitable to provide long-term benefits to materials with a substantial prior heat treatment history is desirable.